1. Field of the Invention
The present invention generally relates to a magnetic sector mass spectrometer and, more particularly, to a mass spectrometer with a magnetic sector incorporating pole pieces which serve both the magnetic sector and provide the magnetic field of an ion-type vacuum pump.
2. Description of the Prior Art
In a mass spectrometer (MS) with a magnetic sector, whether the MS is of the single focusing or double focusing type, two magnetic fields are generally required. One magnetic field is required for the magnetic sector through which the ion beam passes in the gap between pole pieces of the magnetic sector. It is this magnetic field which causes the beam to turn, with the amount of turning depending upon the mass to charge ratio of the ions in the beam. The other magnetic field is required for the operation of the ion-type vacuum pump, which is generally used to evacuate the path in the MS through which the ion beam travels. The elements required to produce the two magnetic fields represent the major portion, in terms of weight and size, of the entire MS.
In a typical prior art small magnetic sector MS, the ion beam travels inside a non-magnetic stainless steel duct. The duct extends from the ion source, such as an ionization chamber, and in case of a double focusing type MS through an electric sector, and therefrom through the magnetic sector to the ion detector. It is this duct which the ion-type vacuum pump evacuates. Since the duct passes through the magnetic sector, the gap between the pole pieces of the magnet, producing the magnetic field, has to be wide enough to accommodate the duct therein. The gap has to be wider than the ion beam height by at least twice the duct wall thickness, plus required assembly clearances.
It is well appreciated that the greater the gap between the pole pieces, the more magnetic material is needed for a desired flux density across the gap. More magnetic material represents increased weight and size of the magnetic sector. For a miniature portable MS the increased weight and size of the magnetic sector represent significant disadvantages. Therefore, for such a MS, the elimination of the duct is highly desirable, since without the duct, gap size can be greatly reduced thereby reducing the weight and size of the magnetic material required to produce the required flux density across the gap. Also, for a miniature portable MS, it is desirable to eliminate the need for separate magnetic material, in order to produce the magnetic field for the ion type vacuum pump, thereby further reducing the overall size and weight of the MS. Furthermore, in such a portable MS, it is desirable to locate the pump as close as possible to the magnetic sector in order to eliminate the need to shield parts of the MS, which may be sensitive to a stray magnetic field, from the magnetic field of the pump, if the latter is located away from the magnetic sector.